This invention relates to solid flue gas desulfurization sorbents and their preparation, and to methods of removing sulfur oxides from gas mixtures using the same.
Sulfur dioxide is an atmospheric pollutant which is present in small amounts in various waste gas mixtures, such as flue gas and certain smelter gases. Flue gases may also contain small amounts (usually only trace quantities) of sulfur trioxide. Sulfur dioxide and sulfur trioxide will be referred to collectively herein as "sulfur oxides". Processes for the selective removal of sulfur oxides from flue gas and other waste gas streams are known. Most of these processes are cyclic regenerative processes employing either a solid sorbent or an aqueous solution which selectively removes sulfur oxides.
Examples of cyclic regenerative processes using a dry solid sorbent or acceptor are described in British Pat. Nos. 1,089,716, 1,154,009, and 1,160,662, and in U.S. Pat. No. 3,501,897. British Pat. Nos. 1,089,716 and 1,160,662 describe the use of copper oxide on gamma-alumina as the sorbent; British Pat. No. 1,154,009 discloses the use of potassium oxide and vanadium pentoxide on porous alumina; and U.S. Pat. No. 3,501,897 discloses both types of sorbents. In the processes of all of these patents, flue gas containing SO.sub.2 and oxygen is contacted with the solid sorbent or acceptor until breakthrough of SO.sub.2 into the effluent gas occurs. The sorbent is then regenerated with a reducing gas. Removal of SO.sub.2 is accomplished in these processes by reaction of SO.sub.2 and oxygen with the active component of the sorbent; thus, copper oxide is partially converted to copper sulfate. The sulfation of the active materials in nearly all cases is incomplete at the time that breakthrough occurs. Breakthrough may be defined as occurring when a stated percentage of the SO.sub.2 in the incoming gas, e.g., 10% over a whole cycle, passes into the effluent gas. Other contact masses, such as copper oxide on silica have also been tried out but found to be less satisfactory than copper oxide on alumina.
Although gamma-alumina is a good carrier material from the standpoint of sorbent activity in flue gas desulfurization sorbents, it has been found to be subject to attrition, even in fixed beds, after numerous flue gas desulfurization cycles, as reported for example in British Pat. No. 1,160,662. To improve the hardness and attrition resistance of the sorbent, British Pat. 1,160,662 suggests the use of about 1 to 20% by weight, calculated on the solid carrier material, of colloidal silica as a reinforcing material.
The preparation of coated or reinforced high surface area catalysts for other processes is well known. U.S. Pat. Nos. 3,502,595 and 3,615,166 are cited as two examples of such catalysts and their methods of preparation. U.S. Pat. No. 3,502,595 describes a process of preparing cracking catalyst having an acidic support such as silica-alumina or titania-alumina by reacting gamma alumina which is at least slightly hydrated with an alkyl ester, such as ethyl orthosilicate, in an organic solvent medium, and separating the solid alumina-inorganic oxide particles from the solvent and the alcohol produced in the reaction. U.S. Pat. No. 3,615,166 describes coated catalysts comprising a high surface area refractory core material (e.g., alumina) and a coating oxide (e.g., zirconia or thoria), which are prepared by dispersing colloidal size particles of the core material in an aqueous solution of a zirconium or thorium salt, adding an alkaline reagent to precipitate zirconia or thoria, drying and calcining. The products of both patents, U.S. Pat. Nos. 3,502,595 and 3,615,166 can be impregnated with other catalytically active metals (e.g., platinum) by known techniques. Reactions of metal alkoxides (which can also be considered as alkyl esters of inorganic acids) are also discussed in D.C. Bradley, "Metal Alkoxides", ACS Monograph No. 23, "Metal Organic Compounds", pp. 10-37 (1959).